There is provided a cooling structure of a power transmission device 1 that the power transmission device includes a tubular input shaft 3 and an output shaft 4R that is inserted into and disposed inside the input shaft 3, and that the power transmission device delivers power output from an electric motor (drive source) 2 to the input shaft 3 to the output shaft 4R via a deceleration mechanism T and a differential device D, wherein the output shaft 4R is disposed such that the output shaft 4R is offset relative to a shaft center of the input shaft 3 in a radial direction, and an oil supply pipe 12 is disposed in an axial direction in a wide space that is formed on a side opposite to an offset side of the output shaft 4R in a space S between the output shaft 4R and the input shaft 3.